The invention relates to a process for compensating for a magnetic interference field in a vehicle.
In order to direct the route of a vehicle, it is necessary to determine the direction of the earth's magnetic field in relation to the vehicle. The magnetic field is measured by a magnetometer, e.g. a magnetic field probe, which is fixedly mounted in the vehicle. During a calibration run, e.g. a circular run, the parameters of the locus curve, namely of an ellipse, of the magnetic field are determined in accordance with a known process. The parameters of the elliptical locus curve are the two semiaxes a and b, their rotation through a specified angle .delta. which describes the shape, and the displacement from the coordinate origin. This displacement is designated the "interference field vector".
An analysis of the accumulated measurement data of the magnetic field probe has revealed that the interference field vector changes in a discontinuous manner on account of external influences, while the shape of the locus curve as a rule remains constant. Such influences are, by way of example, the movement of a steel sliding roof or of a glass sliding roof with a steel frame, changing slopes of the roadway which extend perpendicular to the direction of travel, the loading or unloading of metallic loads in the vehicle or the switching of an electrical load in the vehicle, such as for example the rear window heating. These changes are longer-lasting or take place slowly, for example in the range of minutes. They must be distinguished from short-term interference or interference fields, which are caused by subways or buses traveling past.
Accordingly, various compensation processes have already been described. By way of example, German reference DE-OS 36 44 681 discloses a navigation process for vehicles having an electrical compass, in which strong magnetic influences in the environment of the magnetic field sensor are compensated for using a dynamic drag process. In the known process, a weighted interference field vector is added to the interference vector. In this process, it is however disadvantageous that the distinction between short and longer-lasting interference fields takes place only exclusively with the aid of the weighting factor. If the weighting factor according to the known process is selected to be 0.1 with a cycle time of 100 msec, then much short term interference, as mentioned above, is fully involved in the interference vector displacement and thus corrupts the result of the navigation in a lasting fashion. If, on the other hand, the weighting factor is selected to be so small that the above effect does not occur, then the short term interference fields are nevertheless involved in the result, but more weakly. Moreover, the drag then lasts for a very long time, for example in the ten-minute range, so that navigation is subjected to interference for an unreasonably great length of time.